A technique for the in situ localization of DNA polymerase activity in acrylamide gels has been developed and used to show that the DNA-synthesizing complex moves more slowly then free DNA polymerase and is assayable with a native DNA template whereas free DNA polymerase is not. A new factor has been discovered which is essential for the formation and stability of the complex. It is heat-labile and co-migrates on Biogel A 1.5 m with a DNA-stimulated ATPase activity. It has been found that hydroxylapatite and ECTEOLA-cellulose can be used to purify further both the complex and the new factor.